I've been using the first preview and 15.10.1. I have to say I'm surprised how many major bugs there are in the basic functionality. I've been mostly using Excel and PowerPoint, and out of those Excel is way more buggier.
Powerpoint has suddenly lost all images from the master slide, replaced with broken red crosses that can't be removed. Also it has suffered from saving problems - suddenly it says the file is somehow locked, then trying to save with a different name you get just "Document not saved" dialog box, that is it.
Excel has numerous bugs in very basic usage.
-Scrolling is still quite difficult, if you try to pull from a cell corner downwards I usually jump several thousands of lines at ones
-Filter row is broken, for instance it doesn't show different values are selectable from the column
-Freeze pane is broken, sometimes it is impossible to scroll the first rows after the freeze pane into view with mouse, only by keyboard, sometimes the first row of freeze pane gets duplicated.
-Copy&paste a cell containing a formula is unreliable! I was just yesterday dragging-from-corner a cell with a formula, it would fill the formula. Then cmd-C, cmd-V elsewhere.. it worked.. but randomly it would quit pasting the formula and start pasting the value! Have to manually check every cell that I paste.
I've reported these all to MS via the feedback system, but I'm really boggled at how basic Excel functionality can be so broken. It is as if it was intentional so that people could not use the preview version